When all else fails read the owners manuel. You can either listen to people on here, listen to the oil manufactures or listen to the engineers who designed the engine. These people have no motivation to sell you anything and they know the most about the engine.
My owners manuel for a 91 Prostar 190. It has a 351 indmar, much like yours. It says 30w.
That's it. No 10w-30 or 5w-30. Just straight 30w oil.
Unless you are using your boat in temps below freezing you don't need the 10w-30.
In your lawn mower, motorcycle etc..... use straight 30 weight unless you will be using in temps below freezing. In temps that get down in the teens and 0's, use 5w.
Do a google for oil vescosity. It is very interesting. I did it last year and learned a bunch.
Makes feel much more educated. It made me understand what kind of oil to use instead of just guessing.

Boyd, again, no oil expert here, but oil weight aside, I've seen a lot of people on the boat forums expressing concern over the lowering of zinc additives in many of the oils traditionally popular in the boats. This is something that has happened in the last few years. For folks with flat tappet cams like you and me, the consensus is that it is something we don't want to go without. Many have mentioned Chevron Delo, Royal Purple or Valvoline VR1 as brands that have maintained acceptable ZDDP levels for our motors. I know there are others, but I can't call them up right now. If straight 30 weight oil works for you, great. But check those ZDDP levels, too.

East, I"ll check it out. I'm not an oil expert, just a P.E. teacher who can read an owners manuel. Heck, I don't even know what a flat tappet cam is.
I also don't know what ZDDP is. I do realize and understand that they have probably learned alot about oil and engines since 1991 and they would probably add to their manuel if they could.
Take care. I'll check into zinc and see what they say.
BOYD

Check it out. Good information. Go to the bottom and you will see the amount of zinc in the oil.
The article talks about if you need zinc and how much is needed. Also what the "W" represents and should you use an oil with different weights etc ....

Check it out. Good information. Go to the bottom and you will see the amount of zinc in the oil.
The article talks about if you need zinc and how much is needed. Also what the "W" represents and should you use an oil with different weights etc ....

Enjoy,
BOYD

That article is over 10 years old. Good info, but the zinc content is unreliable because it was only the last few years that the EPA mandated lower levels of zddp in oil because zinc build up on catalytic converters was causing poor performance.

That article is over 10 years old. Good info, but the zinc content is unreliable because it was only the last few years that the EPA mandated lower levels of zddp in oil because zinc build up on catalytic converters was causing poor performance.

That's the key. For more than you ever wanted to read about the subject, you can hunt down an oil thread from about 2007 on www.correctcraftfan.com. The horse was pretty well beat to death.

There was a later thread over here, too. From that, it was somewhat settled that the MC engines that would need to be worried about these things are any Fords and the pre-vortec TBI Chevies.